1. Field of the Invention
The present invention relates to an atomising nozzle and to such a nozzle with a filter, notably to one which produces a spray of fine droplets suitable, for example, for the administration of a medicament by inhalation, to the production of such nozzles, and to a spray generating device incorporating such nozzles.
2. Description of the Prior Art
It is known (e.g. from WO 91/14468) that fluids can be caused to form very fine droplets on being forced through narrow nozzles at high pressure. WO 91/14468 proposes to manufacture the necessary nozzles using methods such as those known in the manufacture of spinning nozzles. These nozzles are produced, for example, by boring through a thin metal plate with a tungsten-carbide needle. An important area of application for the equipment according to WO 91/14468 is the production of aerosols for inhalation therapy. Demanding requirements are imposed among other things, on the fineness of the droplets; it has been found during numerous investigations that a considerable number of droplets must have a size less than 6 .mu.m in order that a sufficient quantity of the medicine can reach deep enough into the lungs. For safe treatment the individual pieces of equipment must each produce the same droplet spectra, since only then is it certain that the given dose of the medicine will be delivered to the lungs in the desired way.
With the mechanical production of nozzles there are sometimes disturbing deviations from nozzle to nozzle, possibly due to the walls of the nozzles being of varying degrees of roughness. It is, amongst other things, difficult to produce double nozzles, like those shown in FIG. 8 of the afore-mentioned WO 91/14468, with the necessary accuracy. In addition, it is not an easy matter to obtain nozzles of changing cross-section using known methods, possibly with a view to accelerating or slowing down the flow of fluid in the nozzle, or to provide impact elements or vortex-generating devices.
In PCT Application No GB91/00433, there have been described methods and devices for forming sprays of fine droplets from a fluid without the use of pressurized propellant gasses, notably for the formation of sprays of a fluid medicament which have a mean droplet size of less than 10 micrometres for inhalation by a user so that the droplets of medicament can penetrate into the lower lung. In PCT Application No GB91/02145, there have been described methods and devices by which the formation of such sprays can be optimised by inducing secondary flows in the stream of fluid when it passes through the nozzle aperture.
In the preferred form of such methods and devices, a metered dose of the fluid medicament is drawn from a reservoir into a pressure chamber by retracting a piston in a cylinder of a pump mechanism against the action of a drive spring. The piston or spring is latched or otherwise retained in the retracted, or cocked, position so that the metered dose is held at ambient pressure in the pressure chamber of the pump until it is discharged. When discharge is required, the piston or spring is released and the spring drives the piston forward, thus applying a rapid pressure rise to the fluid causing it to discharge through the nozzle aperture and form a spray of droplets.
The very fine droplets required for the application of a medicament to the lower lung are achieved by the use of fine aperture size nozzles and high pressures, typically with nozzle apertures of less than 20 micrometres and pressures in excess of 300 bar.
The nozzle apertures required to achieve such fine droplets can be formed in a number of ways, for example by punching a hole in a metal plate and part closing up the hole to achieve a fine aperture with a rough rim which causes the secondary flows in the fluid stream as it passes through the nozzle aperture. However, the techniques used to form the nozzle aperture either require accurate machining of components on a microscopic scale, which is expensive and time consuming and does not give consistent results, leading to rejection of components during quality control assessment prior to use or to inconsistent operation of the device. Furthermore, the need to be capable of enduring the very high pressure surge, possibly as high as 600 bar, when the device is actuated requires the use of mechanically strong components. Again, this adds to the cost of the device.
In PCT Application No GB91/02147 there has been described a form of construction which incorporates an integral one way valve and filter in the nozzle assembly to prevent air being sucked into the device through the discharge nozzle when the piston is being retracted to draw the metered dose of fluid from the reservoir and to prevent blockage of the fine nozzle aperture by solid particles entrained in the fluid. In a preferred form of such a construction a cylindrical plug is a push fit in a chamber immediately upstream of the nozzle orifice to provide an annular passage between the internal wall of the chamber and the radially outward wall of the plug. This annular passage has a radial dimension equal to or less than the nozzle aperture and thus provides a fine filter to remove solid particles which might otherwise block the nozzle aperture. The fine annular passage also imposes a flow restraint on the movement of fluid which is overcome by the high pressure generated when the piston is driven on its forward, or discharge, stroke to allow fluid to flow outwardly through the nozzle aperture. The flow restriction, however, prevents fluid from flowing back into the device as the piston is retracted. This reduces the risk of contamination of the fresh fluid drawn into the pressure chamber from the reservoir with air or fluid from the nozzle assembly downstream of the plug. Again, such a device must be manufactured from metal to be able to withstand the pressure surge as the device is operated and thus requires high precision machining of components which is expensive.